My neighbour had a party last night that went onto 3.30am.

Question

The music was very loud and it kept us awake. My neighbour doesn't want to talk about it. He left his teenagers at home and it was their party. The trouble is, he often goes away and leaves them behind. I'm in the UK.

Answer 1

bring it up casually to the teens, and let them know it's a neighborhood issue.  Make it sound like the police will be intervening in the future

Answer 2

Next time don't hesitate, call the police.  If you mention it to them beforehand they will know you were the one who called.